Heavy metals and radiogenic isotopes in river systems affected by urban and industrial activity: the Valdinievole sub-basin (Tuscany, Italy) case study

The Usciana River is located in the lower part of Valdinievole, one of western sub-basins of the Arno River Basin (Tuscany, Central Italy). It represents the only emissary of the Padule di Fucecchio, a swampy zone which acts as a collector of all creeks and channels draining the highly anthropized northern part of the basin. This PhD project is aimed at characterizing the natural and anthropogenic contributions of heavy metals in the river system of the Valdinievole sub-basin by analyzing waters, suspended solid loads, and stream sediments. Many relevant human activities and small-medium enterprises, such as paper mill industries, floranursery farms, thermal spas, and one of the most productive Italian tanning districts are located in this area, making it an ideal laboratory to investigate how the anthropogenic pressure affects surface waters. In this work we present preliminary analytical data on waters and suspended solids obtained by Ionic Chromatography (IC) and Inductively Coupled Plasma Mass Spectrometry (ICP-MS) for the determination of major, minor, and trace concentrations, respectively. Radiogenic isotope composition was analyzed by Thermal Ionization Mass Spectrometry (TIMS) on suspended solid (Sr, Nd and Pb) and in water samples (Sr). A relatively large geochemical variability is shown by the water samples, whose composition is affected by seasonal variations, ranging from Ca2+(Mg2+)-HCO3 - to Na+-Cl-(SO4 2-) geochemical facies. The most relevant variations are recorded for the dissolved N-bearing species during high and low river discharge periods, as they varied from 36 to 80 (NO3 -), from 1.2 to 2.4 (NO2 -), and from 12 to 23 (NH4 +) mg/L, respectively. A relatively high concentration of Cs was detected in a few waters and suspended solids where concentrations up to 170 μg/L and 46 mg/kg, respectively, were measured, highlighting a dilution trend along the river system. The Pb isotopic signature of the suspended solids reflects the contribution by anthropogenic sources, especially for those samples collected in the most urbanized and industrialized areas. The Enrichment Factors of Ni, Mo, and Pb are positively correlated with the Pb isotopic ratios since a clear trend between geogenic and anthropogenic sources is evidenced. On the other hand, the Sr isotopic composition of waters mainly reflects selective dissolution of carbonates with respect to silico-clastic rocks. The Nd isotopes are presently in progress.